In amperometric oxygen sensors, which operate in marginal operation, the oxygen supply to the working electrode must be limited by suitable measures because oxygen is inherently present at relatively high concentrations. The reduction may be performed by means of capillaries, channel structures or porous material. The reduction of the oxygen transport is of significance especially in case of portable devices, because oxygen is permanently present and leads to a continuous flow of current in the measuring device, which shortens the service life of the oxygen supply being carried.
An oxygen sensor of the said type has become known from U.S. Pat. No. 4,132,616. The prior-art oxygen sensor contains a working electrode and a counterelectrode, which are accommodated in an electrolyte-filled sensor housing. The gas reaches the working electrode via a capillary, which reduces the measuring gas supply to the working electrode. The capillary is closed with a porous PTFE membrane toward the measuring gas atmosphere. The capillary extends at right angles to the plane of the working electrode and is accommodated in an end cap, which is attached to the sensor housing in the area of the working electrode. The capillary has a length of a few mm.
In gas sensors with round sensor housing, the capillary can usually be integrated without greater problems. However, the length of the capillary is often limited, because certain installation dimensions must not be exceeded, and the possibilities of varying the diameter-to-length ratio are often limited.
However, flat gas sensors of a film-like structure, in which the working electrode is exposed directly to the measuring gas atmosphere via an opening in the film, are known as well.
Such a gas sensor is shown, for example, in DE 195 47 150 C2. Due to the design, diffusion can be limited here only with porous films, and there are only lmited possibilities for affecting the porosity and the transport of substances toward the working electrode. This has a disadvantageous effect on the value and the quality of the measured signal.